exporting earths

[JACKC]

Can anyone explain what is meant by exporting earths and the dangers of doing so. I would have thought the more earthing points the better.

 

[Spark123]

Exporting earths to another building you mean? There are certain requirements for doing this re supply type and eq. bonding requirements.

 

[JACKC]

Yes, in relation to 1 earth from one building and then to another from the same supply

 

[bernardgreen]

Yes, in relation to 1 earth from one building and then to another from the same supply

The problem is that "earth" potential in building [A] may not be the true earth potential ( but still safe for that building ) so exporting it to building could have the earth of building higher than true earth leading to the possibilty of electric shock between the imported "earth" on exposed metal work and the real earth.

 

[JACKC]

So when wiring a shed from a spare mcb is it best to use the rcd from this board or wire shed as a TT

 

[JACKC]

Thanks for answering so quickly, its that an issue of wiring matters said to use a spare mcb and use this supply earth and then other friend mentioned exporting earths. This term confused me.

 

[plugwash]

exporting earths is somewhere where you have to be carefull.

if the building is made of insulating material (e.g. wood) and has no services then there is no problem.

if the building is metal or has substatial metal parts in contact with the ground or a concrete base (concrete is apparently surprisingly conductive, more so than most soils from what i've read) then it really needs to be bonded.

equally if there are any services they also need to be bonded.

if your service is PME and you need to bond something in the outbuilding then afaict the earth to your subbuiling should be up to the specs of a main bonding conductor for your size of PME supply. That means 10mm for domestic sized PME supplies.

 

[Steve]

if the building is metal or has substatial metal parts in contact with the ground or a concrete base (concrete is apparently surprisingly conductive, more so than most soils from what i've read) then it really needs to be bonded.

Would this not depend on its moisture content? I cant imagine dry concrete being conductive. And say I had a shed with a concrete base, and no ECPs, If the concrete were conductive, wouldn't it be seen as an ECP in itself, and therefore require me to sink an earth rod at the shed?

 

[JohnD]

Don't know, but a concrete base without a DPC will not be dry.

 

[JACKC]

Thanks for the info, you never stop learning on this forum.